1. Field of the Invention
This invention generally relates to a lockup device of a torque converter. More specifically, the present invention relates to a lockup device of a torque converter that suppresses drag torque while keeping a sufficiently large torque transmission capacity.
2. Background Information
Torque converters usually include a fluid coupling mechanism for transmitting torque between the crankshaft of an engine and the input shaft of an automatic transmission. Generally, a torque converter can perform smooth acceleration and deceleration because it transmits a power via working fluid. However, an energy loss occurs due to slip of the working fluid, resulting in low fuel efficiency. Accordingly, in recent years, to improve fuel efficiency, some torque converters have included lockup devices that, upon reaching predetermined operating conditions, lockup the torque converters so that power from the crankshaft of an engine is directly transmitted to the automatic transmission, bypassing the fluid coupling device. Upon engagement, lockup devices often cause a shudder, or vibration. Further, while engaged, the lockup device is subject to vibrations caused by sudden acceleration, or deceleration, or other vibration including circumstances associated with internal combustion engines. Consequently, torsional vibration dampening apparatus' are typically employed in lockup mechanisms to dampen vibrations.
A torque converter has three types of runners (impeller, turbine, stator) located inside for transmitting the torque by means of an internal hydraulic oil or fluid. The impeller is fixedly coupled to the front cover that receives the input torque from the power input shaft. The hydraulic chamber formed by the impeller shell and the front cover is filled with hydraulic oil. The turbine is disposed opposite the front cover in the hydraulic chamber. When the impeller rotates, the hydraulic oil flows from the impeller to the turbine, and the turbine rotates. As a result, the torque is transmitted from the turbine to the main drive shaft of the transmission.
As mentioned above, some of the conventional torque converters utilize lockup devices for mechanically coupling a front cover on an input side and a turbine on an output side to directly transmit the torque between the crankshaft of the engine and the drive shaft of the transmission. The lockup device is disposed in a space between the front cover and the turbine to divide the space into a first hydraulic chamber on the front cover side and a second hydraulic chamber on the turbine side. The lockup device is primarily formed of a disk-like piston, a driven plate and torsion springs. The disk-like piston can be pressed against the front cover. The driven plate is attached to a rear side of the turbine. The torsion springs elastically couple the piston to the driven plate in the rotating direction. The disk-like piston carries an annular friction member adhered to a position opposed to a flat friction surface of the front cover.
In the conventional lockup device, the working fluid flowing through the main unit of the torque converter controls the operation of the piston. More specifically, a hydraulic operation mechanism in an external position supplies the working fluid to a space between the piston and the front cover when the lockup device is disengaged. This working fluid flows radially outward through the space between the front cover and the piston, and then flows from its radially outer portion into the main unit of the torque converter. When the lockup device is engaged, the working fluid in the space between the front cover and the piston is drained from its radially inner portion so that the piston moves toward the front cover. Thereby, the friction member arranged on the piston is pressed against the friction surface of the front cover. In this manner, the torque of the front cover is transmitted to the turbine via the lockup device.
The conventional lockup device may employ a multi-disk clutch for using multiply friction plates and thereby multiply friction surfaces because only one friction surface cannot provide a sufficient torque transmission capacity in some cases.
In the multi-disk clutch of the conventional lockup device, contact that causes an undesirable drag torque is liable to occur between the friction plates when the lockup device is released.
In the conventional multi-disk clutch, the two friction surfaces double the transmission torque of the lockup device so that increased wearing, breakage and other problems may occur in the friction members.
In view of the above, there exists a need for a lockup device which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.